It is known that, when a measuring body is set into motion with a low level of mechanical-contact friction, gas molecules impinging upon the surface of this body tend to brake its rotation as a result of friction between the surface, which can be considered to have microscopic roughness, and these gas molecules.
Gas-friction vacuum meters utilizing this principle are known (see Dushman and Lafferty, "Scientific Foundations of Vacuum Technique", Wiley, New York, 1962).
A gas-friction vacuum meter is a measuring instrument for determining pressure especially in the high vacuum range. The measurement parameter for pressure determinations is the braking of a freely rotating body by the impingement of gas molecules on a surface of this body in the evacuated space or in a measuring cell or chamber connected with this space.
To exclude mechanical friction losses, i.e. the braking effect of bearing or contact friction, the rotating body is suspended in a contactless bearing system, preferably a magnetic bearing.
The rotating body can be in the form of a flat disk, a cylindrical or conical element or a ball; preferably it is a magnetically journaled ball.
It is customary for the rotating body to have polished or smooth surfaces. Smooth-surface rotary bodies are characterized by a gas-friction coefficient which is largely independent of the type of gas in the evacuated space. Apparently such smooth or polished surfaces have a microscopic roughness generated by the presence of adsorbates on the surface, which results in diffuse scattering of the impinging gas molecules thereby rendering the braking effect independent of the nature of these molecules but sensitive to the density or number of impinging molecules per unit time, a function of the pressure.
It has been found, however, that the gas friction coefficients of extremely clean adsorbate-free surfaces of rotating measuring bodies as are encountered when such bodies are placed in ultrahigh vacuum environments, have greatly reduced values (see R. G. Lord, Tangential Momentum Accommodation Coefficients of Rare Gases on Polycrystalline Metal Surfaces, Rarefied Gas Dynamics, AIAA, New York, 1977, pages 531 ff.).
Such a reduction in the gas-friction coefficient is a phenomenon associated with a frictionless "reflection" of the gas molecules upon their impingement upon a macroscopically smooth surface of a rotating body which is deficient in microscopic roughness. The gas-surface momentum exchange under these conditions is thus of the "mirror" type and the surface can be referred to as a mirror surface.
Additionally the gas-friction coefficient of the rotating body is affected by macroscopic surface variations. For example, in the case of corrosion or the development of soot deposits upon the surface, the gas-friction coefficient is raised to high levels although the measurement precision decreases sharply since deviations in the gas-friction coefficient can greatly exceed the measurement precision of about 1% with instruments normally used.